Alias frame (B) for subplot

Include test to check that map frame setting is applied to all subplot figures when using 'begin' subplot directive.

pygmt/base_plotting.py

@@ -884,7 +884,7 @@ def legend(self, spec=None, position="JTR+jTR+o0.2c", box="+gwhite+p1p", **kwarg
             lib.call_module("legend", arg_str)
 
     @fmt_docstring
-    @use_alias(F="dimensions")
+    @use_alias(F="dimensions", B="frame")
     def subplot(self, directive: str, row: int = None, col: int = None, **kwargs):
         """
         Manage modern mode figure subplot configuration and selection.

pygmt/tests/test_subplot.py

@@ -22,6 +22,21 @@ def test_subplot_basic():
     return fig
 
 
+@pytest.mark.mpl_image_compare
+def test_subplot_frame():
+    """
+    Check that map frame setting is applied to all subplot figures
+    """
+    fig = Figure()
+    fig.subplot(directive="begin", row=1, col=2, dimensions="f6c/3c", frame="WSne")
+    fig.subplot(directive="set", row=0, col=0)
+    fig.basemap(region=[0, 3, 0, 3], frame="+tplot0")
+    fig.subplot(directive="set", row=0, col=1)
+    fig.basemap(region=[0, 3, 0, 3], frame="+tplot1")
+    fig.subplot(directive="end")
+    return fig
+
+
 def test_subplot_incorrect_directive():
     """
     Check that subplot fails when an incorrect directive is used